Why do we need URIs for XML namespaces?

Question

I am trying to figure out why we need URIs for XML namespaces and I cannot find a purpose for that. Can anyone brighten me a little showing their use on a concrete example?

EDIT:

Ok so for instance: I have this from w3schools

<root
xmlns:h="http://www.w3.org/TR/html4/"
 xmlns:f="http://www.w3schools.com/furniture">

 <h:table>
   <h:tr>
     <h:td>Apples</h:td>
     <h:td>Bananas</h:td>
   </h:tr>
 </h:table>

 <f:table>
   <f:name>African Coffee Table</f:name>
   <f:width>80</f:width>
   <f:length>120</f:length>
 </f:table>

 </root>

So what should http://www.w3schools.com/furniture hold ?